Cross-flow turbines with circular cross-section are commonly known and have been widely used in hydropower applications around the world for many decades.
This type of turbine operates on a free-jet principle, utilizing a nozzle to direct high velocity fluid flow through a runner. This runner can consist of multiple blades of circular arc section arrayed about a single axis, comprising in their entirety a cylindrical shape. Water flows through the cylinder in a direction perpendicular to the cylinder axis, so that the fluid performs work on two different areas of the circumferential blades. By design, this type of turbine extracts most of the kinetic energy in the flowing water within the two stages of the turbine, and because the exhausted water has little remaining velocity, this kind of turbine does not require a draft tube to ensure high conversion efficiency. Draft tubes can be utilized on some vertical-outflow cross-flow type turbines, to allow a suction effect and enable capture of the full gross head of a hydropower plant.
Circular cross-flow turbines have a number of advantages and benefits compared to other turbines, such as Francis turbines. Circular cross-flow turbines are capable of maintaining a relatively high efficiency over a wide range of flow rates, something that Francis turbines cannot do. The cross-flow runner is self-cleaning, because the leading edge of the first stage becomes the trailing edge in the second stage. Most cross-flow turbines are designed with bearings outside the water path, reducing the risk of environmental contamination.
However, circular cross-flow turbines have a number of limitations. Their efficiency, peaking between 70%-85% depending on the design and care taken in manufacture, is lower than conventional reaction turbines such as Kaplan or Francis turbines, as well as other impulse turbines such as Pelton turbines, all of which can attain efficiency greater than 90%. Circular cross-flow units suffer lower efficiency due to several causes. Only a small fraction of the circular cross-flow runner circumference is actually usable as cross-flow area. A significant fraction of the flow through the runner becomes entrained in the blades and is exhausted without fully contributing to the turbine's shaft work. Additionally, in theory the flow streamlines crossing through the runner between stages must actually cross each other, and under some conditions, this stream impacts the central axle or shaft, causing additional losses. Furthermore, most practical cross-flow runners require multiple support ribs along the length of the blade. Each of these ribs creates areas of local turbulence and efficiency loss.
The form factor of a circular cross-flow turbine is such that the runner diameter is strongly related to the unit's flow capacity, and thus the unit's power rating. Because the runner diameter is directly related to the shaft speed, there is a limit to the practical size, and thus flow rate, of this type of unit. Typical cross-flow turbine shaft speeds are relatively slow, especially for large-flow units at low head. Cross-flow turbines are intolerant of being operated if the runner comes into contact with the lower water level. At many low head sites, the tail water level can vary substantially (by several meters), leading to difficult plant design with this kind of turbine.
In addition to the common circular cross-flow turbine, a variety of linear crossing-flow machines are known. In the device disclosed in U.S. Pat. No. 7,645,115, flow passes through two stages of blades in a direction perpendicular both to the blades' path of travel, as well as to the axes of the two parallel axles supporting the drive belts or chains. The blades in U.S. Pat. No. 7,645,115 are designed such that their curvature is symmetric about the path of travel, unlike the design of the current invention. The reaction force on the blades in U.S. Pat. No. 7,645,115 is ideally in-line with the path of blade travel, but under certain operating conditions, such as when not operated at the optimal blade-to-water speed ratio, substantial drag loads perpendicular to the path of blade travel can force the moving blades inwards towards the array of stationary guidevanes. To prevent damage from collision due to unwanted inward deflections, the drive belts in this kind of linear turbine must be highly tensioned. This tension exceeds the minimum required tension for normal power transmission, reduces powertrain life, and imposes additional stress on the machine.
Hydropower plants must be designed to operate safely even if the utility grid connection is lost. Normally, in the event of power loss, the turbines must be quickly shut down to prevent risk of damage due to high speed operation. High-flow turbines, such as Kaplan, bulb, circular crossflow, or Francis turbines, are subject to large pressure fluctuations (known as water hammer), if the turbine is suddenly turned off, or if a grid-disconnect event occurs and the machine rapidly accelerates. This is because all the water flowing through these types of turbines must be stopped, to fully depower the turbine. Pelton turbines, used only at sites having very high pressure, benefit by being able to use a jet deflector plate to divert the water stream away from the turbine in an emergency, which allows fast and safe shut-down without water hammer, because only the direction of flow is changed, not the rate of flow.